1. Field of the Invention
The present invention relates to a method for growing a nitride semiconductor crystal on a sapphire substrate, and a nitride semiconductor light emitting device produced by such a method.
2. Description of the Related Art
Presently, nitride semiconductors are used for ultraviolet to blue lasers. Conventionally, a nitride semiconductor crystal is usually heteroepitaxially grown on a mirror finished sapphire (0001)-plane substrate. As the crystal growth method, a vapor phase growth method such as an MOCVD method or an MBE method is used.
In the MOCVD method, for example, a sapphire (0001)-plane substrate is processed under a reduced atmosphere at about 1000.degree. C., and ammonia is supplied for initial nitridation of the substrate surface. Then, a GaN or AlN buffer layer is formed at a low temperature of about 500.degree. C., and the temperature is increased to about 1000.degree. C., thereby producing a nitride semiconductor crystal.
Japanese Laid-open Patent Publication No. 9-23036 discloses a technique which improves optical characteristics by a 2-step growth method via a buffer layer with the angle between the sapphire substrate surface and the C-face being about 5.degree. or less. However, Japanese Laid-open Patent Publication No. 9-23036 only discloses a technique to improve the luminance of a light emitting device, and does not disclose a technique to improve the flatness of the uppermost surface of a crystal or to reduce the crystal dislocation density thereof. Both the flatness and the crystal dislocation density influence the production process of a light emitting device and the characteristics of the produced light emitting device.
In the above-described MOCVD method, the crystal growth involves an initial nitriding process and a low temperature growth of a buffer layer. This method, however, results in a high dislocation density and an insufficient crystal quality which raise problems particularly for a laser device operated at a high current density. Moreover, simply setting the inclination angle of the substrate surface to about 5.degree. or less does not sufficiently define the step density on the substrate surface, i.e., does not provide a sufficient flatness of the surface. Therefore, the crystal quality and the surface condition of the nitride semiconductor may vary sensitively to variations in the substrate inclination angle.
With an inclination angle ranging from about 0.2.degree. to about 5.degree. as disclosed in the prior art, the nitride semiconductor, e.g., GaN, grown on a (0001)-plane sapphire substrate has dislocations therein at a high density, which are generated from the interface with the sapphire substrate. It is believed that this occurs for the following reasons: three dimensional growth is likely to occur due to a difference in lattice constant between the sapphire substrate and GaN; and the use of the sapphire substrate with an inclination angle ranging from about 0.2.degree. to about 5.degree. with respect to the &lt;0001&gt; orientation results in a non-uniform step distribution on the substrate surface which in turn inhibits a well-regulated step flow growth.
In the heteroepitaxial growth, the buffer layer formed at a low temperature is initially in an amorphous or polycrystalline state, and the atoms therein are then rearranged while increasing the temperature so as to form into a single crystal while receiving information from the substrate. At a high temperature, the buffer layer has an effect of allowing for a crystal growth under conditions similar to those of a homoepitaxial growth. However, in a system with a substantial lattice mismatch such as sapphire and GaN, the effect of the buffer layer is insufficient, and the conditions for forming the buffer layer for absorbing the step difference on the substrate surface due to the inclination angle with respect to the &lt;0001&gt; orientation are limited to narrow ranges.
As does the buffer layer, the initial nitriding process on the substrate surface also has an effect of allowing for a crystal growth under conditions similar to those of a homoepitaxial growth. However, the nitriding process is controlled by the detachment of oxygen atoms near the sapphire surface and the diffusion of nitrogen atoms from the vapor phase, whereby the nitriding process proceeds while remaining the initial shape of the substrate surface. Thus, the steps on the sapphire substrate surface having an inappropriate inclination angle will remain without being substantially affected by the nitriding process, which in turn inhibits a step flow growth and creates dislocations.
FIG. 1 is a schematic microscopic view of such a substrate.
A substrate 101 has a non-uniform distribution of steps 102 on the order of atomic layers. After reaching the substrate surface, a material species undergoes repeated migrations and re-detachments and then reaches a stable site on the substrate surface, i.e., the atomic layer step 102, where it forms a growth nucleus 103. However, the growth nucleus formation also becomes non-uniform reflecting the non-uniform step distribution. Therefore, in a growth process on a substrate with an ill-defined inclination angle, a two dimensional growth (a mode in which each layer is deposited in a plane) is inhibited, and the three dimensional nucleus growth (a mode in which the crystal growth proceeds in a localized manner) becomes dominant, thereby causing a threading dislocation at a location where adjacent grown crystals are coupled with each other.